The past few years have seen enormous growth in the field of fiber optic communications. The distances over which reliable communications operate have been increased, as have the bandwidths and the number of installed systems. Wavelength division multiplexing (WDM) is a method by which many messages can be sent simultaneously at different optical wavelengths with interference only between simultaneous messages at the same wavelength. WDM has greatly increased the traffic capacity of an optical fiber link, and local area network (LAN) configurations have been developed. Most of these systems have been developed for civilian or nontactical military communications for which the probability of a link outage is remote. Thus, most LANs employ star, ring, or bus architectures.
Task 1 of The Westinghouse single mode fiber optical LAN (SMOLAN) began by examining single mode fiber optic communications architectures for tactical LANs. LANs using star, ring, or bus architectures cannot provide the redundancy required for reliable communications in a tactical military situation in which links are subject to damage from the movement of military equipment and vehicles as well as enemy action (artillery, air strikes, etc.). In addition, these architectures cannot provide the flexibility required for communications with dispersed, frequently relocating units or elements. Irregular grid networks were selected as the architecture most suitable for providing this service.
Grid and gridlike networks can be evaluated by examining a simple two-way grid as shown in FIG. 1 of U.S. Pat. No. 5,128,789 issued to Irwin Abramovitz, and incorporated by reference herein. The present invention is a variation of the system disclosed therein. Such a grid can be assembled to cover, for example, a 10 km.times.10 km area in which dispersed units will connect to the nearest node each time they relocate. Several routing techniques were investigated and evaluated. A highly desirable technique is to "flood" the network, providing the message to all nodes, so that the system need not know at which node a particular unit is located. Thus, no configuration or reconfiguration following a unit's move is required. The grid network provides a large number of redundant paths, ensuring a very high probability that a path will exist even in the face of significant network damage (many links out). It is this desirable multiplicity of paths, however, that creates the most severe obstacle to implementation of such a grid network: severe signal degradation and inter-symbol interference. This problem results from the fact that all such "shortest paths" will not be of identical length. There are longer multiple paths as well.
The solution to this problem is twofold: (1) a switching algorithm that does not allow multiple paths yet takes advantage of path redundancy when damage occurs, and (2) a functional node design to implement that algorithm. This routing-by-"flooding" algorithm offers advantages: because all nodes will receive the message, the system does not need to know where a node is located (except for each node knowing the units attached to it); thus, no configuration or reconfiguration is required. Further, no central control is required (a central control would make the network vulnerable to the loss of the controller and/or add the complexity of backup controllers). In other words, switching should take place based upon information normally passing through the node. Such an algorithm in a system using WDM would accept only one message at a time (the first one to arrive) at each wavelength from among the several inputs to the node and would further pass the same message only once; if a message appeared a second time it would be stopped. The accepted inputs would be passed along to all attached nodes. A message would have a finite lifetime within the network because, after a given time, it would have reached all nodes so no node would be passing it. Therefore, the list of messages (message IDs) already passed can be purged of old messages and would not grow to a significantly large number. This switching algorithm is, in fact, self-routing and finds the shortest path between sender and receiver, and is discussed in the aforesaid patent to Abramovitz and herein.
A preliminary architecture has been developed under Task 1 for a node designed to implement the required node functions. This node design is shown in FIG. 3 of the aforesaid patent for one of several node input lines. Other node input lines would use identical hardware, sharing a common controller and joining the output of this hardware at an N-to-one coupler as shown.
Considering only one of several input fibers to the node, the input fiber is split, providing signal to both an optical spectrum analyzer (OSA) and a fiber optic delay (several feet of optical fiber). The OSA determines the wavelengths present. If on/off keying is used to transmit the message header information, the OSA may even detect the message ID number. OSA output goes to the controller which determines which wavelengths are to be passed (and which are to be stopped). A common controller accepts and processes information from the several OSAs, each associated with an input to the node. The controller implements the aforesaid switching algorithm as previously described with these requirements:
1. If a wavelength is present on more than one input, pass only the signal that appeared first. PA1 2. Do not pass the same message more than once.
The delay holds the optical signals long enough for the OSA and controller to set up the appropriate optical switches. The OSA could be grating or other angularly dispersive device that separates the input signals by wavelength, lenses, and photodetector array. Following the delay, the optical signals are again demultiplexed by an angularly dispersive device similar to that for the OSA. Since these spatially separated wavelengths or channels are not far enough apart, and lack gaps in between, in order to couple to a monolithic set of on/off optical switches, focal plane dissector techniques are used to route or fan out these optical signals into an array of switches.
Under the direction of the controller, the various optical channels are switched by an array of parallel optical switches. Next, inverse star couplers are used to bring together groups of channels for multiplexing. These channels must be multiplexed into several groups of limited optical bandwidth since optical amplifiers have limited gain-bandwidth products. Following amplification, the optical signals may be multiplexed into one full-bandwidth fiber and summed with similar signals from the other inputs to the node in an N-to-one coupler. This combined output may then be split in a one-to-N coupler and sent out to the attached nodes.
Such a node allows implementation of a SMOLAN using WDM, potentially incorporating up to 500 wavelengths (channels), each modulated to 20 GHz, thus providing a system information bandwidth on the order of 10 GHz.
Wavelength division multiplexing (WDM) was selected over alternative multiplexing schemes because of the large number of channels available in the 1.2-to 1.6 micrometer band, its compatibility with the selected routing technique and switching algorithm, and its potential use together with packet switching. Code division multiple access (CDMA) and time division multiplexing (TDM) are not compatible with the switching algorithm described above since different simultaneous messages cannot be readily separated to permit the passing of some and the stopping of others.
System considerations can influence required device parameters in other ways as well. In a source-driven WDM system, each receiver has its own unique wavelength; the transmitter sends its message at the receiver's wavelength.
Thus, a receiver may use a narrow-band laser local oscillator (in a coherent system requiring a local oscillator), while a transmitter must have a tunable laser, although the tuning speed need not be particularly fast (perhaps many milliseconds).
On the other hand, in a sink-driven system, for which each transmitter has its own unique wavelength, the receiver must tune to each transmission intended for it. Therefore, a transmitter may use a narrow-band laser, and a receiver must use a tunable laser. This tunable laser, however, must tune very rapidly (on the order of tens of nsec) so that the beginning of incoming messages will not be lost. Optical fiber delays of tens of nanoseconds (requiring tens of feet of optical fiber) may be used to hold the message during laser tuning, but larger delays are impractical.
At a high traffic site such as a headquarters, it may be desirable to provide for multiple simultaneous messages by using several receivers there. In a source-driven system, several fixed wavelengths would be assigned. However, a transmitter would not necessarily know which of those wavelengths are in use. In a sink-driven system, each receiver there would tune to a different transmitter, and a similar problem would not develop.
Due to its high bandwidth potential and other factors, single mode fiber optical communications are being considered within the military community for tactical local area networks. In a tactical environment, star, ring and bus architectures may be replaced by grid or grid-like networks in order to provide a large multiplicity of paths to ensure reliable communications. It is this large number of paths, however, which creates a severe multipath problem resulting in intolerable signal degradation and intersymbol interference. Thus there is a need for a unique node design which addresses this problem.
The solution to this multipath problem involves a switching algorithm which does not allow multiple paths yet takes advantage of path redundancy when damage occurs, and a node design to implement that algorithm. The advantages of a "flooding" algorithm has been previously mentioned, i.e., since all nodes will receive the message, a node need not know where a unit is located (except those units directly attached) and thus no configuration or reconfiguration is required. Further, it is desirable that no central control be required since that would make the network vulnerable to loss of the controller and/or add the complexity of back-up control. In other words, the switching takes place based upon information normally passing through the node in accordance with the aforesaid switching algorithm. Such an algorithm in a system employing wavelength division multiplexing many messages may be sent simultaneously on different wavelengths with interference only between simultaneous messages at the same wavelength would accept only one message at a time, the first one, at the same wavelength from among the several node inputs, and would further pass a message only once, i.e., if a message appears a second tie it would be stopped. The accepted inputs would be passed along to all attached nodes. Since a message would have a finite lifetime within the network (after a given tie it will have reached all nodes so no node will be passing it), the list of messages already passed (message ID's) may be purged of old messages and would not grow to a significantly large number.
Ideally, the node would be able to handle a large number of simultaneous wavelengths, for example, 500 wavelengths (channels) each modulated to 20 GHz bandwidth between 1.2-1.6 micrometers. This would ordinarily require an extremely complex node, providing a communications capability far in excess of near-term needs. A more practical near-term node, limited in the number of simultaneous wavelengths, is the subject of this invention.